Chemical conversion coating aluminum has been used by airframe and other manufactures, including the military, to improve aluminum's corrosion resistance and paint adhesion properties.
The aluminum alloy with chromate conversion coatings such as AA2014, AA2024 and AA7075, etc., that are generally used on military and aerospace equipment must comply with the regulation of MIL-C-5541 and MIL-C-81706, in which the corrosion resistance test of ASTM B117, salt spray test is applied. The specified corrosion resistance cannot be less than 168 salt spray hours.
There have been many patents and reports on how to get the chemical conversion coating aluminum to easily pass military regulation. But, how to decrease the concentration of chromate solution without affecting the corrosion resistance of the chemical conversion coating aluminum has not been reported yet.
The U.S. Pat. No. 4,451,304 of Bateuk claims a method for treating deoxidized aluminum prior to conversion coating which improves the ability of the aluminum to pass rigid corrosion resistance requirements. The method uses either a chromic or non-chromic deoxidizer to get rid of aluminum alloy oxide film. The preferred deoxidizer used in this process is hydrofluoric acid or a salt that contains fluorine ion. The chemical conversion coating solution used is Alodine 1200S (manufactured by Amchem Products, Inc.) that consists of mainly fluoride. Although the method does not use a chromate compound as the deoxidizer, the contents of chromate on the chemical conversion solution used is not decreased.
U.S. Pat. No. 4,614,607 of Loch claims a new composition of deoxidizer that can meet the military standard regulation on the corrosion resistance of aluminum alloy without using ammonium nitrite. The main components of the composition are ammonium nitrate, ammonium bifluoride(AMB), sulfuric acid, and nitric acid. The above composition can process metal by soaking. Silica can also be added to obtain a sticky form of deoxidizer, to be used on partial process of specimens that are not suitable for complete soakage. However, the patent does not report anything regarding the decrease of chromate content of the chemical conversion solution.